The present invention relates to rotary drill bits including a plurality of conically shaped rotary cutters and specifically to one wherein the cutters are rotatably supported in at least two series of balls on a trunnion of the drill bit body.
Rotary drill bits are previously known wherein each trunnion is provided with an annular shoulder adjacent the drill bit body and the trunnion is fastened by means of trunnion bolts in the bore of the annular shoulder. The row of balls near the drill bit body runs in a roller path groove formed in part by the annular shoulder and in part by a bearing inner ring mounted on the trunnion bolt. A second series of balls is arranged further away from the drill bit body and runs in a roller path groove formed in part by the bearing inner ring and in part by an enlarged portion at the end of the trunnion bolt. By reason of this construction, the trunnion has a relatively low strength characteristic because of the resultant weakening due in part to the incorporated trunnion bolt with the result that in normal applications the roller drill bit is not able to effectively withstand high impact stresses and in many cases breaks even at normal operating conditions. Breakage of course causes considerable damage. Furthermore, in this type of assembly the rotary cutters are not adequately secured against being displaced from the trunnion by reason of the fact that the trunnion bolt under normal operating conditions tends to work loose and give way as the result of high trunnion stresses. Lastly, it is noted that manufacture and assembly of the rotary drill bit described above is rather expensive and may be termed uneconomical because of the precise and accurate machining required of some of the mating parts.